He became involved in the Anti-Apartheid Movement while still at school and continued to organise Anti-Apartheid activity as a student politician. In 1976 he travelled to South Africa to meet local campaigners and learn about the apartheid system first hand. During this visit he was subject to surveillance by the South African Police and eventually decided to leave the country due to the risk this posed to the local anti-apartheid campaigners. He then went on to serve on the National Executive Committee of the Anti-Apartheid Movement from 1979 to 1994 when free elections were held in South Africa.[1]

He was the Chair of Sheffield District Labour Party from 1993 to 2009, resigning the position in order to fight the Sheffield Central at the 2010 general election, after long-serving MP Richard Caborn announced he would be standing down. He has been a long-standing opponent of tuition fees.[4] Blomfield became the Chair of Labour for Democracy, or Labour4Democracy, on 4 December 2012. The internal party group was launched to promote plurality in the Labour Party, and increase co-operation with other political parties in building a "progressive consensus".[5][6][7][8][9]

Blomfield underwent surgery to remove a benign brain tumour, a meningioma, in August 2011, going on to make a full recovery. This tumour could have been developing for 20 years. It was diagnosed when it began pressing against his optic nerve and interfering with his sight. He was diagnosed at the Royal Hallamshire Hospital within a week of noticing symptoms. The operation, by Mr Thomas Carroll lasted 13 hours and was back at work after 4 weeks.[13] He and Conservative MP Guy Opperman, who also had a brain tumour in 2011, walked the first section of the Pennine Way in 2012 to raise money for Headway UK.[14]